Fractional differential equations (FDEs) are a generalization of the classical ordinary and partial differential equations, in which the order of differentiation is permitted to be any real (or even complex) number, not only a natural number.
FDEs containing not only one fractional derivative but also more than one fractional derivative are intensively studied in many physical processes [1]}, [2]}, [3]}. Many authors demonstrate two essential mathematical ways to use this idea: multi-term equations [4]}, [5]}, [6]}, [7]}, [8]} and multi-order systems [9]}, [10]}.
